1. Field of the Invention
The present invention relates generally to the art of pyrolytic deposition of metal oxide films, and more particularly to the pyrolytic deposition of metal oxide films from aqueous solutions of coating reactants.
2. The Prior Art
The pyrolytic deposition of metal oxides onto a glass surface is described in U.S. Pat. No. 3,660,061 to Donley et al. Organometallic salts, preferably acetylacetonates, are dissolved in an organic vehicle such as methylene chloride. Other suitable solvents include aliphatic and olefinic halocarbons, halogenated hydrocarbons, alcohols, and nonpolar aromatic compounds such as benzene and toluene. The organic solution is sprayed onto a hot glass surface where it thermally decomposes to form a metal oxide film which changes the reflectance and transmittance of solar energy by the glass.
In U.S. Pat. No. 3,441,934, Englehart et al disclose that cobalt oxide is particularly suited for use as a coating on glass because of its good solar heat shielding properties. However, cobalt oxide coatings are not as uniform and adherent as desired. Englehart et al teach that the adherence, durability and optical quality of cobalt oxide films are improved by providing the glass surface with a thin tin oxide undercoating, and then overcoating with cobalt oxide by thermal pyrolysis of a cobalt compound such as cobalt acetylacetonate. Preferably, the cobalt oxide coating also contains the oxides of iron and chromium since these oxides improve the scratch resistance of the coating without detracting from the solar heat shielding properties of the cobalt oxide. A preferred solvent system for the metal coating reactants comprises a polar organic compound and a nonpolar aromatic compound, particularly methanol combined with either benzene or toluene.
Current interest in eliminating the health and environmental hazards of using large volumes of organic solvents has encouraged the development of aqueous coating compositions. It is known from U.S. Pat. No. 2,688,565 to Raymond that light reflecting coatings of cobalt oxide may be deposited by contacting a hot glass surface with an aqueous solution of cobalt acetate. However, such films have a grainy, irregular texture and are highly porous, resulting in poor acid resistance evidenced by debonding of the film.